Flammable gas concentration measurements are made in a number of safety-critical situations. One such flammable gas is natural gas, which typically comprises mainly methane, plus higher hydrocarbons, inert gases and trace components. Natural gas detectors are needed for a number of applications including response to public reported gas escapes and for continuous monitoring of plant/equipment using permanently installed detectors. They are required to measure the gas concentration as a percentage of the lower explosion limit (LEL) of the gas mixture, this being an important safety parameter.
The required accuracy of gas detectors is given by international standards. For example, European Standard EN 50057:1999 specifies that the instrumental accuracy of such instruments shall be no more than either ±5% of the measuring range (usually 0–100% LEL) or ±10% of the reading, whichever is the greater. Some published company standards demand an even greater level of accuracy. None of the published standards for natural gas leak detectors specifically addresses the issue of measurement errors that arise from compositional variation in the gas, and base their requirements on the error when the detector is faced with a consistent composition. However, it is desirable to meet the required standards of accuracy even with gas compositions of inconsistent composition.
While methane-specific detectors (ie detectors with no cross-sensitivity to any other gas) have been used for determining the safety of natural gas mixtures, such devices do not have the necessary cross-sensitivity to other flammable components and can give large errors.
The most extensively used detectors are based on pellistor sensors or flame ionisation technology. Pellistor-based sensors work by oxidising the gas mixture on a heated catalyst bead. This provides a measure of combustibility that is inherently related to the % LEL of the natural gas leak. However, there are a number of associated problems with these sensors when detecting natural gas leaks:    i) The cross-sensitivity to different flammable gases is not exactly what is required to compensate for their differing LELs. Correction factors must be applied if the sensor is used to measure a gas species different from the gas used to calibrate it.    ii) So-called “foldback” of the sensor response, in that, if the gas is present in concentrations sufficient to displace large quantities of oxygen from the air, the gas cannot burn in the detector and its response actually decreases as the gas concentration increases. This can result in ambiguity of the reading in safety-critical situations.    iii) A high cost of ownership resulting from a high maintenance requirement.
A number of gas-specific detectors, mostly based on specific detection of methane, have been developed to overcome these difficulties for natural gas applications. Many use optical detection of the gas, and in particular measure the optical absorption of the methane or other gases at specific wavelengths in the infrared region of the electromagnetic spectrum. Examples include European patent specification 874233 A2 (Siemens AG). Methane is chosen because it is the principal component of natural gas. However, the other components of natural gas cause errors in the resulting % LEL reading. Because these components have a varying composition, it is not sufficient to calibrate out the systematic error; the remaining random error is still too large to meet the demands of the detector standards mentioned above.